1. Field of the Invention
The present invention relates to a laser marking method for creating a matrix type two-dimensional bar code.
2. Description of the Related Arts
Recently there begin to appear signs of prevalence of two-dimensional bar codes having information in two-dimensional directions. The two-dimensional bar codes include a stack type bar code consisting of one-dimensional bar codes which have vertically been stacked to display information by its vertical and horizontal representations, and a matrix type bar code consisting of black and while cells which have been arranged in a mosaic manner (matrix form) in vertical and horizontal directions to display information.
FIG. 14 illustrates Data Code which is a typical standard of the matrix type.
The Data Code comprises L-shaped guide cells or border cells consisting of black cells arranged continuously so as to form an L, timing cells consisting of white and black cells alternately arranged opposite the border cells, and a data area consisting of white and black cells arranged inside the border cells and the timing cells in an arbitrary pattern based on the display data.
In order to read the two-dimensional bar code of this type, the entire bar code is fetched as an image by a CCD camera and is interpreted using an image recognition technique. Through the image processing, in this case, the position and the direction of turn (angle) of the two-dimensional bar code are recognized on the basis of the border cells and the coordinates of each cell are determined relative to the timing cells. Then, the pattern of array of the white and black cells lying within the data area is rearranged into the original code.
In order to recognize each cell CE in this image recognition, it is identified whether a predetermined point (representative point) RP within the cell CE is black or white as shown in FIGS. 15A to 15C. Then, on the basis of the result of identification, decision is made of whether it is a white cell or a black cell. Depending on the number of the representative points RP, there are for example a one-point method (FIG. 15A), a four-point method (FIG. 15B) and a five-point method (FIG. 15C).
On the other hand, various printing methods are feasible for creating two-dimensional bar codes. Among them, a laser marking method has an advantage in that two-dimensional bar codes can directly be printed on the surface of a workpiece, and hence is useful for the direct marking onto semiconductor wafers, IC packages, etc.
In a conventional typical laser marking method for the creation of the two-dimensional bar codes, as shown in FIG. 16, a beam spot BS of laser beam is traversed horizontally across the surface of the workpiece to obtain one scanning line HS, and then the horizontal scanning line HS is displaced vertically by a predetermined pitch d to iterate the scanning action predetermined number of times. Ordinarily, several lines are allocated to cells in a line.
Accordingly, each black cell CE.sub.B is a unit region in which several horizontal scanning lines HS are juxtaposed in the vertical direction. On the other hand, the white cell CE.sub.W is a unit region in which such horizontal scanning lines HS are absent (jumped over) without being plotted.
Incidentally, in case a single black cell is formed from a single laser beam spot having a larger diameter, the cell center may inconveniently be deeply recessed, making it difficult to recognize images.
In FIG. 16, to facilitate the illustration, the traces (scanning lines) of the laser beam spot are shown in an intermittent manner at a pitch equal to the spot diameter. Actually, however, the traces are commonly in the form of continuous straight lines.
The two-dimensional bar code as described above is supported by an error correction function so as to ensure a correct restoration in spite of 20 to 30% of lacks of the data area. However, this error correction function is feasible only by the entire bar codes, which does not mean that rough display contents (black/white) of individual cell is permitted. It must definitely be displayed whether the individual cell is a black cell or a white cell.
In the case of the above conventional laser marking method of this type, such a demand for the cell display quality could be met by increasing the density of the horizontal scanning lines HS.
However, as compared with the time (the time of portions indicated by solid lines or broken lines of FIG. 16) during which the beam spot BS scans within each black cell CE.sub.B, each horizontal scanning line HS requires a longer time (the time of portions indicated by one-dotted chain lines) in order to move the beam spot BS from a terminal end (right-hand end) of each black cell CE.sub.B, over the region of the white cell CE.sub.W by skipping, to a start end (left-hand end) of the adjacent black cell CE.sub.B. In addition, such skipping action is repeatedly carried out over each white cell CE.sub.W.
Furthermore, to prevent the marking quality from being degraded at the opposite ends of the bar code, as shown in FIG. 17, there is also performed a switching operation of the horizontal scanning lines on the outside of the ends, which results in an unnecessary waste of time.